In general, a particle beam therapy system is provided with a beam generation apparatus that generates a charged particle beam, an accelerator that is connected with the beam generation apparatus and accelerates a generated charged particle beam, a beam transport system that transports a charged particle beam that is accelerated by the accelerator so as to gain predetermined energy and then emitted, and a particle beam irradiation apparatus, disposed at the downstream side of the beam transport system, for irradiating a charged particle beam onto an irradiation subject. Particle beam irradiation apparatuses are roughly divided into apparatuses utilizing a broad irradiation method in which a charged particle beam is enlarged in a dispersion manner by a scatterer, and the shape of the enlarged charged particle beam is made to coincide with the shape of an irradiation subject in order to form an irradiation field; and apparatuses utilizing a scanning irradiation method (the spot-scanning method, the raster-scanning method, and the like) in which an irradiation field is formed by performing scanning with a thin, pencil-like beam in such a way that the scanning area coincides with the shape of an irradiation subject.
In the broad irradiation method, an irradiation field that coincides with the shape of a diseased site is formed by use of a collimator or a bolus. The broad irradiation method is a most universally utilized and superior irradiation method where an irradiation field that coincides with the shape of a diseased site is formed so as to prevent unnecessary irradiation onto a normal tissue. However, it is required to create a bolus for each patient or to change the shape of a collimator in accordance with a diseased site.
In contrast, the scanning irradiation method is a high-flexibility irradiation method where, for example, neither collimator nor bolus is required. However, because these components for preventing irradiation onto not a diseased site but a normal tissue are not utilized, there is required a positional accuracy of beam irradiation that is the same as or higher than that of the broad irradiation method.
In recent years, in order to treat a complex-shape diseased site, the demand for the degree of freedom in forming a beam has become large. It is required to apply the scanning irradiation method to a craniocervical portion, because that portion includes major organs such as the eyeballs, the optic nerve, the spinal cord, the brain, and the like. Unlike a body portion, the size of a craniocervical portion is small; therefore, the depth to a diseased site is relatively small and hence the necessary beam energy is small. FIG. 17 represents the relationship between the energy of a charged particle beam and the beam size thereof. The abscissa denotes the beam energy E (MeV) of a charged particle beam, and the ordinate denotes the beam size S (mm) of the charged particle beam. The beam size is calculated in such a manner as a standard deviation is calculated. In FIG. 17, the beam size S denotes a beam size at an isocenter in water. A characteristic 92 denotes a beam size that is a physical limit caused by water scattering; a characteristic 91 denotes a beam size at a time when a charged particle beam that has been launched through the beam extracting window of a conventional particle beam irradiation apparatus passes through the air and enters the body of a patient. Because the irradiation characteristic of a radiation in a human body is almost the same as that of a radiation in water, an aquatic irradiation characteristic is examined.
For example, the range of a proton beam of 150 MeV is approximately 16 cm when the loss in a beam extracting window, a position monitor, the air, and the like is neglected; in many cases, the range of a craniocervical portion is shorter than the range of the proton beam. In other words, in the case of a conventional technology, even though considering the case, a particle beam having as low energy as 150 MeV and a small size, as represented in FIG. 17, is required, a particle beam of as low energy as 150 MeV is affected largely by scattering (angle) in a beam extracting window, a position monitor, and the air caused before the particle beam enters water; thus, the beam size thereof becomes extremely large. As a method of reducing the beam size, it is conceivable to reduce the distance between a material that scatters a charged particle beam and a to-be-irradiated body (diseased site).
Patent Document 1 discloses an invention in which in a particle beam therapy system utilizing a scanning irradiation method that requires a high accuracy in the beam irradiation position, an obstacle that causes beam scattering is placed at a position that is as downstream in the beam as possible so that the beam size is reduced. The invention disclosed in Patent Document 1 is provided with a beam scanning apparatus that scans a charged particle beam, a first duct in which a beam extracting window is provided at a position that is at the downstream side of the beam scanning apparatus, an irradiation apparatus that makes a charged particle beam pass through the first duct and that irradiates the charged particle beam onto an irradiation subject, a second duct, and a beam transport apparatus that makes a charged particle beam, launched from an accelerator, pass through the second duct and that transports the charged particle beam to the irradiation apparatus; a beam position monitor (referred to simply as a position monitor, hereinafter) that measures the position of a charged particle beam is mounted in the beam extracting window through the intermediary of a holding member; a vacuum region in the first duct and a vacuum region in the second duct communicate with each other.
The first duct includes two ducts; the two ducts are airtightly bonded with each other by use of a bellows. By use of a duct driving means and a duct expansion/contraction means that expands and contracts the first duct in the beam-axis direction, the bellows is expanded and contracted and the position monitor, which is provided at a position that is in the vicinity of and at the downstream side of the beam extracting window, is moved in the beam-axis direction of the duct, so that the air gap between a patient and the beam extracting window is suppressed from becoming unnecessarily large and hence the beam size is reduced.